Jun-Ki Chung

Effects of deposition rate and thickness on the properties of YBCO films deposited by pulsed laser deposition

YBCO films with various thicknesses from 100 nm to 1.6 µm were deposited on single crystal SrTiO3 substrates by pulsed laser deposition (PLD). The effects of thickness and deposition rate—by means of controlling the pulsed laser frequency—on the critical current density (Jc) were studied. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to examine the orientation, crystallization and surface quality. The amount of a-axis YBCO component evaluated from the ratio of XRD chi-scan intensities of the a-axis and c-axis for the YBCO (102) plane increased as the YBCO film became thicker. SEM was used to analyse the surface of YBCO film, and it was shown that the surface of YBCO film became rougher with increasing thickness. There were many large singular outgrowths and networks of outgrowths on the surface of the YBCO films with thickness greater than 0.4 µm. The increased amount of a-axis YBCO component and the coarse microstructure of the thick YBCO film caused degradation of Jc with increasing thickness.

YBCO coated conductor using biaxially textured clad composite Ni–Mn/Ni–Cr substrate

A new biaxially textured composite tape of Ni?4.5%?Mn/Ni?1.5%?Cr was used as a substrate for a YBCO coated conductor through the RABiTS approach. Multi-layer CeO2/YSZ/Y2O3 buffer layers and YBCO film were deposited on the substrate by pulsed laser deposition. The deposition conditions of the buffer layers and the YBCO were studied and compared. Good biaxial textures have been obtained for buffer layers on composite Ni?4.5%?Mn/Ni?1.5%?Cr substrates. Scanning electron microscopy on sample cross-sections was used to examine the interface and diffusion of oxygen. The uniform formation of an Ni?Mn?O layer between NiO and the Ni?4.5%?Mn layer was observed, and the Ni?Mn?O layer restricted the further growth of NiO layer, which was thin and not continuous, within the coated conductor during YBCO deposition at higher temperature and higher oxygen pressure. The Jc of YBCO films on these metal substrates was 1.5 ? 106?A?cm?2 at 77?K, 0?T.

Fabrication of meter-long coated conductor using RABiTS-PVD methods

The practical application of HTS materials in power devices requires the HTS conductor which is long, powerful (high current carrying), and affordable. Many different methods for making practical coated conductor include multiple ways of obtaining texture templates and a variety of film deposition methods which include both physical and chemical routes. In this work, three film deposition systems (pulsed laser deposition, sputtering, and evaporation) equipped with reel-to-reel metal tape moving apparatus were installed and used to make meter-long coated conductor. Buffer architecture of CeO/sub 2//YSZ/Y/sub 2/O/sub 3/ was continuously deposited on Ni alloy using sputtering, evaporation, and PLD. YBCO superconducting layer was continuously deposited on buffered metal tape by pulsed laser deposition. End-to-end critical current I/sub c/ of 80 A and critical current density J/sub c/ of 1.3 MA cm/sup -2/ for 600 nm-thick YBCO at 77 K, self-field have been achieved in 1 cm-wide tape over 1 meter length. Longitudinal distribution of the I/sub c/ values was between 72 A/cm and 90 A/cm.

Tunable Properties of Ferroelectric Thick Films With MgO Added on (BaSr)TiO 3

MgO enhanced (Ba 0.6 Sr 0.4 ) TiO₃ thick films have been fabricated by a tape casting and firing method for tunable devices on the microwave frequency band. In order to improve ferroelectric properties, the composite thick films enhanced with MgO on BST have been asymmetrically annealed by a focused halogen beam method. Dielectric constants of composite thick films are changed from 1050 to 1300 at 100 ㎑ after 60 sand 150 s annealing by the focused halogen beam. Even though no prominent changes were previously observed from the thick films before and after annealing in terms of chemical composition and surface morphology, it is clear that the average particle size of the thick films calculated by Scherrers formula were increased by annealing. Furthermore, a strong correlation between particle size and dielectric constant of the composite thick films has been observed; dielectric constant increases with increased particle size. This has been attributed to the increased volume of ferroelectric domain due to increased particle sizes. As a result, the tuning range was improved by halogen beam annealing.

SYNTHESIS AND CHARACTERIZATION OF MgB2 FILMS DEPOSITED BY ULTRASONIC SPRAY PYROLYSIS

The MgB2 films have been prepared on the c-cut sapphire substrates by ultrasonic spray pyrolysis (USP) technique using Boric acid and Magnesium acetate tetrahydrate dissolved into distillated water and ethanol as starting materials. The overall concentrations were fixed at 0.1mol/L. During the deposition, 96% Ar - 4% H2 was used as carrier gas. The reaction temperatures were 850 °C, 900 °C and 950 °C . The deposited MgB2 films showed strongly (1 0 1) plane orientation. In the sample deposited at 950 °C for 5min, MgB2 film showed a homogeneous surface, with good grain connectivity. The critical temperature (Tc) was observed to be 31.5 K. The crystal structural and microstructural properties of the MgB2 films prepared by USP were observed to be strongly dependent on the reacting temperature.

STUDY OF MICROSTRUCTURE OF HYDROCARBON DOPED MgB2 WIRES BY TRANSMISSION ON ELECTRON MICROSCOPY

The microstructure of the MgB2 wires with hydrocarbon, pyrene (C16H10) addition has been investigated by transmission electron microscopy (TEM) equipped with energy-dispersive X-ray spectroscopy (EDX). In our study, MgB2 wires have been fabricated through powder-in-tube (PIT) process using pure Fe as sheath material and sintered at 600°C ~800°C for 30min to 4 hrs under high purity argon gas. TEM and EDS analyses were conducted on the specimens prepared by tripod polishing and ion milling method. The structural features such as second phases, interface and defects were investigated by electron microscopy.

CHARACTERISTICS OF MgB2 THIN FILMS SYNTHESIZED BY CO-EVAPORATION METHOD

MgB2 thin films have been synthesized by co-evaporation method by controlling the relative deposition rates of B and Mg on sapphire (Al2O3) substrate. Before the co-evaporation of Mg and B, the deposition rates of each element have been measured separately with various substrate temperatures. As the substrate temperature increased above the optimum growth condition, Mg started to evaporate from the already reacted MgB2 and form MgB12 as a second phase, which resulted in the tendency decreasing the critical temperature. Superconducting properties have been measured by PPMS and the microstructural features have been characterized with XRD, SEM and EDS.

Study on Depositing Oxide Films on Ni Substrate for Superconducting Tape

High temperature superconducting coated conductor has a structure of ///. The buffer layer consists of multi-layer, this study reports the deposition method and optimal deposition conditions of YSZ(Yttria-stabilized zirconia) layer which plays a important part in preventing the elements of substrate from diffusing into the superconducting layer. YSZ layer was deposited by DC reactive sputtering technique using water vapor for oxidizing deposited elements on substrate. To investigate optimal thickness of YSZ film, four YSZ/CeO/Ni samples with different YSZ thickness(130 nm, 260 nm, 390 nm, and 650 nm) were prepared. The SEM image showed that the surface of YSZ layer was getting to be rougher as YSZ layer was getting thicker and the growth mode of YSZ layer was columnar grain growth. After CeO layer was deposited with the same thickness of 18.3 nm on each four samples, YBCO layer was deposited by PLD method with the thickness of 300 nm. The critical currents of four samples were 0, 6 A, 7.5 A, and 5 A respectively. This shows that as YSZ layer is getting thicker, YSZ layer plays a good role as a diffusion barrier but the surface of YSZ layer is getting rougher.

Fabrication of Coated Conductor by Continuous PVD Methods

Continuous physical vapor deposition (PVD) method is one of many processes to fabricate long length coated conductor which is required for successful large-scale application of superconducting power devices. Three film deposition systems (pulsed laser deposition, sputtering, and evaporation) equipped with reel-to-reel(R2R) metal tape moving apparatus were installed and used to deposit multi-layer oxide thin films. Both RABiTS and IBAD texture templates are used. IBAD template consists of CeO(PLD)/YSZ(IBAD) on stainless steel(SS) metal tape, and RABiTS template has the structure of CeO/YSZ/YO which was continuously deposited on Ni-alloy tape using RR evaporation and DC reactive sputtering in a deposition system designed to do both processes. 0.4 m-long coated conductor with Ic(77 K) of 34 A/cm was fabricated using RABiTS template. 0.5 m and 1.1 m-long coated conductor with Ic(77 K) of 41 A/cm and 26 A/cm were fabricated using IBAD template.